This invention relates to the extrusion of foamed polymer compositions and more particularly to a process for rapid aging of a foamed polystyrene.
In the manufacture of polystyrene foam, the method wherein heated and pressurized polystyrene resin is mixed with an appropriate foaming agent and then extruded to a low pressure zone is well-known and widely used. Eventually the foam stock is subjected to a thermoforming step in the manufacture of articles such as foam cups, meat trays, snack food containers, etc. A variety of organic blowing agents have a solvating effect on polystyrene. From the stand point of plasticizing the polystyrene during the melt-mixing of the same, this has beneficial effects in that the melt can be more easily worked and mixed during the stage or stages prior to extrusion thereof. If the solvating effect is too great, deficiencies are encountered during subsequent thermal forming of the polystyrene foam. This is due to the strong affinity of such blowing agents for the polystyrene. Thus, after the foam is extruded, these blowing agents tend to remain in the cell walls of the polystyrene and, even after aging for up to several days, the polystyrene foam will tend to sag or distort prior to or during thermoforming. Therefore, the foam material produced by the above method must be aged carefully, to permit evaporation of the blowing agent down to a specific level, or more importantly to allow dilution with air so that sagging and warping during thermoforming is avoided.
It is an object of the invention to provide a process which permits better control over residual plasticizing-type blowing agent remaining in the foamed polystyrene after extrusion thereof.
It is yet another object of the present invention to provide a process which yields a foamed polymer of comparatively lower density.
It is still another object of the present invention to provide a process which reduces the aging time for a styrene resin prior to thermoforming the same.